1. Field of the Invention
The present invention relates to a tank venting system and to a method for diagnosing the tank venting system.
2. Description of the Related Art
Present internal combustion engines have tank venting systems in which fuel that has evaporated in the tank is stored in an activated carbon filter which is connected to the intake manifold of the internal combustion engine via a closable tank vent valve. When the tank vent valve is open, air is drawn in via a connection of the activated carbon filter to the environment, entraining the temporarily stored fuel and supplying it for combustion. The quantity of gas that is drawn in is controlled via the tank vent valve in such a way that on the one hand the activated carbon filter is sufficiently purged with air, and on the other hand, unacceptably large disturbances of the fuel/air ratio of the mixture supplied to the internal combustion engine do not occur.
In order to comply with regulatory requirements, a defective tank vent valve that is installed in a tank venting system must now be recognized as defective via suitable diagnoses. It is already known to open a tank vent valve during operation of the engine and to evaluate a response from a fuel/air ratio control loop for the diagnoses. The fuel vapor from the tank venting (regeneration gas) mixed with air causes a disturbance of the control loop, so that the occurrence of the disturbance indicates properly functioning tank ventilation, and thus in particular a properly functioning tank vent valve. This is described in published German patent application document DE 100 43 071 A1, for example.
It may be provided to repeatedly open the tank vent valve and to use a statistical analysis of a change in the mixture, which results from activating the valve and which is detected with the aid of a lambda sensor, for the diagnosis. This check may be made during idling and also during partial load operation of the internal combustion engine. In the process, the tank vent valve is slowly activated in a ramp-like manner without the overall system taking this activation into account. In other words, during the activation of the tank vent valve, the air and fuel components which are supplied to the engine via the tank vent valve are not taken into account. An intact or defective tank vent valve may be deduced via the response of the overall system to this occurring disturbance variable, which represents a mixture deviation. However, in the event that the mass flow which is conducted via the tank vent valve now contains no hydrocarbon molecules, i.e., when an unloaded activated carbon filter is “purged,” this does not result in a response from the lambda control after the tank vent valve is opened, and therefore it cannot be determined whether a defective tank vent valve is present. In other words, although the tank vent valve is properly activated, it is possible that no mixture deviation is determined, so that, as mentioned, a conclusion cannot be drawn concerning the functioning of the tank vent valve.
Charge detection is carried out with the aid of a hot film air mass sensor, for example. Charge detection with the aid of an intake manifold pressure sensor is also known. In this type of charge detection, the gas which is additionally introduced into the system and thus into the intake manifold via the tank vent valve is measured with the aid of an intake manifold pressure sensor.
In recent hybrid vehicles, the internal combustion engine is continually started and stopped. This makes it necessary to carry out the required diagnostic method for the tank venting system as quickly as possible so as not to hinder other diagnoses.